There are many applications for devices and/or methods to measure the depth of a fluid. For example, it may be desirable to determine the level of liquid fuel in a fuel storage tank, such as the amount of gasoline or diesel fuel in an automobile or watercraft fuel tank. As another example, it may be desirable to determine the depth of water in a storage tank, a swimming pool, a lake, or a river.
Often, the fluid whose depth is to be measured is not static. For example, liquid fuel in an automobile or watercraft fuel tank may slosh around inside the fuel tank in response to movement of the automobile or watercraft. As another example, there may be waves on the surface of a lake.
There are a number of traditional techniques of measuring a fluid's depth. One technique is to use a float/sending unit combination. The float, which floats on the fluid's surface, is connected via a member, such as a rod, to a fixed swivel point on the sending unit. The swivel point is in turn connected to a variable resistor within the sending unit. The resistance of the variable resistor varies in response to a change in the position of the swivel point. Because the float rests on the fluid's surface, the float moves vertically in response to a change in the fluid's depth. The change in the float's vertical position is transferred to the sending unit's variable resistor via the member and the swivel point. Consequently, the variable resistor's resistance changes in response to a change in the fluid's depth. An indicator subsystem, such as an electrically operated fuel gauge, may be connected to the variable resistor and indicate to a user the fluid's current depth.
The float/sending unit combination suffers from a number of disadvantages. First, movement of the fluid to be measured may result in inaccurate measurements. For example, if a watercraft is moving rapidly through water, fuel in its fuel tank may slosh around inside the fuel tank. If the fuel level is measured via a float/sending unit combination, the float may move in response to sloshing of the fuel in the fuel tank. Consequently, the float/sending unit combination may provide an inaccurate fuel level measurement when the watercraft is moving.
A second disadvantage of the float/sending unit combination is that a person generally must manually adjust the member's length and/or arc in order to calibrate the float/sending unit combination. The float/sending unit combination may be located in an area that is difficult to access, such as in a vehicle's fuel tank. Thus, it may be difficult and/or inconvenient to calibrate the float/sending unit combination. Additionally, movement of the fluid to be measured may exert force on the float/sending unit combination and knock it out of calibration.
Finally, because the float moves in response to a change in fluid depth, the float/sending unit combination must comprise moving parts. A system having moving parts may wear out more quickly than a system not having moving parts. Movement of the fluid to be measured may exert a force on the moving parts, which may accelerate their failure. Additionally, the intended movement of the parts may be impaired by contaminants in the fluid to be measured.
One prior art alternative to the float/sending unit combination is an ultrasonic fluid depth measuring system. Such system comprises a transducer which converts an electrical signal to an ultrasonic wave and vice versa. Such system typically operates by injecting an ultrasonic wave towards the fluid's surface via the transducer. A portion of the ultrasonic wave is reflected by the fluid's surface back towards the transducer. The transducer captures the ultrasonic wave reflected by the fluid's surface. By measuring the amount of time that elapsed between when the transducer injected the ultrasonic wave and when the transducer captured the reflected ultrasonic wave, the system can determine the fluid's depth. Such method of measuring a fluid's depth may be referred to as the pulse-echo technique.
In order to accurately determine a fluid's depth by using an ultrasonic fluid depth measuring system, the speed of the ultrasonic wave in the medium through which it travels (e.g. liquid or air) must be known. Typically, one or more calibration reflectors are placed a known distance away from the transducer. By measuring the amount of time it takes for an ultrasonic wave injected by the transducer to be reflected by a calibration reflector and returned to the transducer, the speed of the ultrasonic wave in the medium through which it travels may be estimated.
Although calibration reflectors may generally provide an accurate method of calibrating an ultrasonic fluid depth measuring system, they may increase cost and/or manufacturing complexity of the system. Additionally, use of calibration reflectors may result in false readings when the liquid to be measured is moving. In applications where accurate fluid level depth measures are critical (e.g. watercraft or aircraft fuel level measurements), inaccurate liquid level depth measurements may result in property damage, injury, or death.
As noted above, it is frequently desirable to measure the depth of a fluid that is moving, such as the depth of fuel in a moving vehicle's fuel tank. Some prior art fluid depth measuring apparatuses, such as the float/sending unit combination, may not provide an accurate fluid depth measurement while the fluid is moving. Other prior art fluid depth measuring apparatuses, such as prior art ultrasonic fluid depth measuring systems, may be unacceptably costly and/or complex. Consequently, what is needed is a fluid level measuring system that may provide accurate measurements when the fluid is moving, that is of relatively low cost and easy to manufacture, that may be used to retrofit an existing fluid level measuring system, and that is simple to calibrate.